1. Field of the Invention
The present invention generally relates to an ultrasonic meter that measures flow velocity and volumetric flow of fluid.
Priority is claimed on Japanese Patent Application No. 2008-326905, filed Dec. 24, 2008, the content of which is incorporated herein by reference.
2. Description of the Related Art
As is generally known, there is a reflection correlation type ultrasonic flow meter that uses a reflection correlation method. The reflection correlation type ultrasonic flow meter measures flow velocity by performing a cross correlation processing on a reflected wave. The reflected wave is generated by an ultrasonic pulse emitted to a fluid and reflected by air bubbles and/or particles in the fluid. The reflection correlation type ultrasonic flow meter calculates volumetric flow based on the flow velocity and flow passage sectional area.
A Doppler type flow meter based on a pulse Doppler method is one of other things. The Doppler type flow meter measures flow velocity based on a frequency shift volume that is a Doppler shift volume of a reflected wave. The reflected wave is obtained by an ultrasonic pulse emitted towards a fluid and reflected by air bubbles and/or particles in the fluid. The Doppler type flow meter calculates volumetric flow based on the flow velocity and flow passage sectional area.
Japanese Patent No. 3669580 discloses an ultrasonic flow velocity profile and flow meter based on a combination of the reflection correlation method and the pulse Doppler method. A peak point of cross correlation value based on the reflection correlation method is a value that shows the flow velocity. A plurality of values with high correlativity appear at regular intervals as a result of cross correlation processing. A plurality of flow velocities based on the reflection correlation method are obtained for every fixed value, which is called “multi peak.” It cannot be defined uniquely which peak point of cross correlation value should be adopted as a value that shows a true flow velocity. The multi peak is avoided by calculating an approximate value of the flow velocity based on the pulse Doppler method first, calculating a cross correlation value based on the reflection correlation method, and determining the true flow velocity from the cross correlation value that is in a range of the approximate value of the flow velocity by the pulse Doppler method.
Japanese Unexamined Patent Application, First Publications, No. 2005-181268 also discloses an ultrasonic flow meter that uses the reflection correlation method.
Upper limit of the flow velocity that can be measured in the pulse Doppler method is lower than that in the reflection correlation method, for the reflection correlation method has no upper limit theoretically. The lower limit in the pulse Doppler method is higher than that in the reflection correlation method when a flow velocity is low because the accuracy decreases. Because the range of the flow velocity that can be measured in the pulse Doppler method is narrower than that in the reflection correlation method, the application of the pulse Doppler method is limited. In the pulse Doppler method, the flow velocity is not acquired when a flow of air bubbles and/or particles contained in the fluid is intermittent.
Because the prior art combines the reflection correlation method and the pulse Doppler method and calculates the flow velocity using each of the methods, the configuration of the device is complex in the prior art.